The present invention relates to a color cathode ray tube, and particularly to a color cathode ray tube having precision main lens electrodes for an in-line type electron gun.
Color cathode ray tubes such as a color picture tube, a display tube, and the like are widely used as a receiver of TV broadcasting or as a monitor in an information processing apparatus for their high-definition image reproduction capability.
The color cathode ray tube of this kind includes a vacuum envelope comprised of at least a funnel having a faceplate carrying a phosphor screen on its inner surface at one end thereof, and a neck connected to the end of the funnel housing therein an electron gun structure for emitting electron beams toward the phosphor screen.
FIG. 15 is a schematic sectional view for explaining the configuration of a shadow mask type color cathode ray tube as one example of a color cathode ray tube to which the present invention is applied. Reference numeral 20 designates a faceplate, 21 a neck, 22 a funnel for connecting the faceplate to the neck, 23 a phosphor screen formed on the inner surface of the face plate to constitute an imaging screen, 24 a shadow mask which is a color selection electrode, 25 a mask frame for supporting the shadow mask to constitute a shadow mask structure, 26 an inner shield for shielding external magnetic fields, 27 a suspension spring mechanism for suspending the shadow mask structure on studs heat-sealed to the inner side wall, 28 an electron gun housed in the neck for emitting three electron beams Bs (.times.2) and Bc, 29 a deflection device for horizontally and vertically deflecting the electron beams, 30a magnetic device for carrying out a color purity adjustment and a centering adjustment, 31 a getter, 32 an internal conductive coating, and 33 an implosion protection band.
In the configuration shown in FIG. 15, the faceplate 20, the neck 21 and the funnel 22 constitute a vacuum envelope. Three electron beams Bc, and Bs.times.2 emitted in a line from the electron gun are horizontally and vertically deflected by magnetic fields formed by the deflection device 29 to scan the phosphor screen 23 two-dimensionally.
The three electron beams Bs, Bc.times.2 are respectively modulated by color signals of red (side beam Bs), green (center beam Bc) and blue (side beam Bs) and subjected to color selection in beam apertures in the shadow mask 24 disposed immediately in front of the phosphor screen 23 to impinge upon a red phosphor, a green phosphor and a blue phosphor of the mosaic three color phosphors of the phosphor screen 23, thereby reproducing a desired color image.
FIG. 16 is a top view of main parts for explaining a structural example of an in-line type electron gun structure used for the color cathode ray tube shown in FIG. 15. Reference numeral 10 designates a cathode, 11 a first grid electrode serving as a control electrode, 12 a second grid electrode, 13 a third grid electrode, 14 a fourth grid electrode, 15 a fifth grid electrode, 16 a sixth grid electrode, 16a a correction plate electrode in the sixth grid electrode 16, 17 an anode, 17a a correction plate electrode in the anode, 18 a shield cup, and 19 insulating supports (only one of two is shown).
In the electron gun, three electron beams generated in a triode constituted by the cathode 10, the first grid electrode 11 and the second grid electrode 12 are accelerated and preliminarily focused by the third grid electrode 13, the fourth grid electrode 14 and the fifth grid electrode 15, focused as desired by a main lens formed between the opposing surfaces of the sixth grid electrode and the anode 17, and they are directed toward the phosphor screen as shown in FIG. 15.
In the electron gun of this type, the fifth electrode 15, the sixth electrode 16 and the anode 17 constituting the focus lens are cup-shaped. Particularly, each of the grid electrode 16 and the anode 17 constituting the final lens has a single opening surrounded by an in-turned rim on mutually facing ends thereof and has a correction plate electrode 16a, 17a therein set back from the mutually facing ends thereof which has an individual aperture therein for each of the electron beams, respectively.
FIGS. 17A and 17B are schematic sectional views for explaining a main lens forming electrode of the aforementioned type electrode gun. FIG. 17A is a sectional view in parallel with the in-line direction of the three beams, and FIG. 17B is a sectional view perpendicular to the in-line direction.
In FIGS. 17A and 17B, the sixth grid electrode 16 has a single opening 16-1 in the end face of the sixth grid electrode 16 opposing the anode 17, surrounded by a rim turned in an axial distance H toward the interior of the sixth grid electrode 16, and has a correction plate electrode 16a having three beam apertures therein corresponding to the number of the electron beams and disposed at a position therein set back a distance d1 from the single opening toward the interior of the sixth grid electrode, and similarly the anode 17 has a single opening 17-1 in the end face of the anode opposing the sixth grid electrode 16 across a spacing g, surrounded by a rim turned in an axial distance H toward the interior of the sixth electrode 16, and has a correction plate electrode 17a having three beam apertures therein corresponding to the number of the electron beams end disposed at a position therein set back a distance d2 from the single opening toward the interior of the anode. The correction plate electrode 17a has an opening for passing a center electron beam and forms passageways for side electron beams in cooperation with the inner wall of the cup-shaped anode 17. A combination of the single openings 16-1, 17-1 and the correction plate electrodes 16a, 17a produces an effectively large diameter electron lens. Japanese Patent Application Laid-Open No. 4-43532 Publication discloses an above-described effectively large diameter main lens formed by provision of oval rims in opposing end faces of a pair of electrodes in the main lens and correction plate electrodes set back from the respective opposing end faces toward the interiors of the respective electrodes.
FIGS. 18A to 18C are schematic sectional views for explaining the shapes of the electrodes for a main lens of the conventional electron gun. Generally, the inner wall of the cup-shaped electrode 16 (17) is formed to have an axially uniform inside diameter (in major and minor axis directions) from the open end A to the opposite end B formed with a rim as shown in FIG. 18A. The opening end A sometimes becomes narrower than the opposite end B after manufacturing process such as drawing as shown in FIG. 18B.
The outside diameters of the correction plate electrode are made substantially equal to the inside diameters of the cup-shaped electrode in major and minor axis lengths. Since the correction plate electrode 17a disposed within the anode 17 is semi-circular or semi-oval at both ends of its major axis, only top and bottom edges of the plate electrode in the minor axis direction are welded to the inner wall of the cup-shaped electrode.
When the correction plate electrode 16a (17a) is inserted into the cup-shaped electrode 16 (17) and fixed by laser weld or the like to a position of a desired set back amount d from the electrode end face to manufacture the electrode as shown in FIG. 18C, if the inside diameter of the cup-shaped electrode is of the shape shown in FIG. 18A or FIG. 18B, it is very difficult to accurately position the correction plate electrode 16a (17a) within the cup-shaped electrode (the sixth grid electrode 16 or the anode 17). Thus, it is difficult to establish the dimension d or to secure the parallelism with respect to the single opening, resulting in deterioration of characteristics of the electron gun.
As described above, in the conventional electron gun structure for the cathode ray tube, the correction plate electrode is welded by laser to a position set back from the rim in-turned internally of the opposing end faces of the cup-shaped electrode, within the cup-shaped electrode of the main lens. Therefore, variations in positioning accuracy of the correction plate electrode are caused by variations in the shape of the open end of the cup-shaped electrode, resulting in an increase of astigmatism of the lens.
There is a further problem in that it is very difficult to adjust the positioning of the correction plate electrode after being assembled and welded.
FIGS. 19A to 19C are schematic sectional views for explaining the shape of the main lens forming electrodes of the electron gun previously proposed by the present inventors, FIG. 19A is a sectional view similar to FIG. 17B illustrating the cup-shaped anode 17, FIG. 19B is a front view of the correction plate electrode 17a to be welded and fixed to the interior of the cup-shaped electrode 17, and FIG. 19C is an enlarged view of main parts of FIG. 19B.
As shown in FIG. 19A, the correction plate electrode 17a is inserted toward the opposite end formed with a rim along the inner wall B from the open end A of the cup-shaped anode 17, and fixed at its edges by laser weld or the like to the position of the set back amount d2. AS shown in FIG. 19B, the correction plate electrode 17a has the beam aperture 17ac for passing a center electron beam end two cutouts 17as for passing side electron beams at both its sides. The cutouts 17as form an electron beam aperture in cooperation with the inner wall of the anode 17.
Recesses 17b are formed by press-forming at the edges of the correction plate electrode 17a which contact the inner wall of the anode 17 when inserted into the anode 17, to reduce friction with the inner wall B and secure ease of assembling. However, when the recess 17b is press-formed in the correction plate electrode 17a, burrs 17d occur as shown in FIG. 19C. If the protrusion L' of the burr 17d is larger than the clearance between the plate electrode and the inner wall of the anode 17, this deforms the anode 17 and the correction plate electrode 17a.
In addition to burrs, variations of outside diameters of the correction plate electrode 17a and inside diameters of the open end of the cup-shaped electrode 17 hinder the ease of insertion of the correction plate electrode 17a into the cup-shaped electrode 17. This difficulty with the insertion and variations of conditions of laser weld change the diameter of the opening in the cup-shaped electrode and the diameters of the apertures in the correction plate electrode which play the most important role in the assembled electrodes. This poses a problem in that characteristics of the electron gun is degraded by the reduced accuracy of the main lens electrode geometry and resultant increased astigmatism such that a cathode ray tube can not provide the desired performance.
There is a further problem in that it is very difficult to readjust the position of the correction plate electrode after it is assembled and welded to the cup-shaped electrode.
The same is true for the assembly of the sixth grid electrode 16 and the correction plate electrode 16a therefor, and the description associated with the problem is omitted.